1. Field of the Invention
The present invention relates to a gamma compensating circuit and, more particularly, to a gamma compensating circuit using the amplification degree of a transistor, which circuit provides a gamma-compensation in a display unit other than a cathode ray tube (CRT) so that the picture reproduced in the display unit may be the same as a real subject without picture distortion.
2. Description of the Prior Art
Generally the CRT is composed of an anode, a cathode, a grid, a heater, etc., and an electron projected into the CRT in the state that a high voltage is applied to a cathode terminal is deflected according to the electronic characteristic, and therefore the picture on the CRT is distorted from the real picture of the subject.
Consequently, in order to improve or eliminate such picture distortion, a TV broadcasting station transmits gamma-compensated signals. Thereafter, the transmitted signals are compensated with a deflection phenomenon (gamma characteristic) of an electron in the CRT and thus are able to reproduce the same picture as the real subject. That is, referring to FIG. 1, the CRT has the gamma characteristic to deflect an electron as shown by curve C in FIG. 1 by virtue of a high voltage applied thereto, and thus a broadcasting station transmits the gamma-compensated signal as shown by curve A in FIG. 1 in consideration of the gamma characteristic of the CRT. And such a gamma-compensated signal from a broadcasting station is converted to a signal having a linear characteristic as shown by curve B in FIG. 1 due to the gamma characteristic of the CRT, thereby to reproduce in the CRT the same picture as the real subject.
However, the gamma-compensated signal from a broadcasting station is not applicable to a display unit, e.g., an LCD projector or AMA projector other than the CRT, and therefore a clear picture cannot be obtained in the CRT. In order to resolve the above-mentioned problem, it is required to provide respective output characteristic curves suitable for various display unit characteristics and for this, a gamma-compensation ROM has been digitally provided and a MAP has also been made to have a gamma reverse compensation characteristic value corresponding to an input signal level in order to convert the input signal level to an output signal level having a linear characteristic.
However, this method has a drawback such that where there are differences in the gamma-compensation values according to the display units, it is impossible to vary the gamma-compensation value accordingly.
Also, a known gamma-compensation circuit disclosed in U.S. Pat. No. 4,480,268 granted to Michitaka Osawa et al., and U.S. Pat. No. 4,589,021 granted to Joichi Sato et al., utilizes the amplification of a transistor and non-linear characteristic of a diode to convert the gamma characteristic, but this conventional gamma-compensation circuit has been defective in that a visibility factor of a picture is apt to be influenced by a picture signal transmitted from a broadcasting station.